Radiation inhibits cell cycle progression and this inhibition includes both an arrest of initiation of DNA synthesis, a G/1 block, and a block in passage through G/2. Arrests in cell cycle progression at the G/1/S and G/2/M interfaces, which occur following DNA damage provide an increased opportunity for repair of radiation damage increasing cell survival. The recent elucidation of the molecular details underlying the regulation of cell cycle progression, especially the transition from G/1 to S phase, suggests that it may be possible to understand mechanistically how G/1 arrest following DNA damage occurs, and also whether modulation of G/1 arrest might change the radiation sensitivity of tumor and normal cells. The p53 gene has a role in the G/1 arrest that follows radiation-induced DNA damage in fibroblasts and human tumor cell lines. Damage of cellular DNA by either radiation or chemical agents is associated with an increase in cellular p53 protein levels, and the arrest of cells in G/1. The RB gene regulates cell proliferation in part by restricting cell cycle progression during G/1. Recognition that RB and p53 both exert important regulatory functions at the G/1/S interface has only emerged recently. Experiments to understand the mechanism by which the G/1 arrest following irradiation is mediated may provide insight into the regulation of G/1 in both tumor and normal cells. We, therefore, propose: 1) to characterize the pattern of expression and activity of known mediators of cell cycle progression at G/1 in irradiated normal and malignant cells in which p53 and RB gene function will be evaluated; and, 2) to identify, isolate, and characterize genes important for the G/1 cell cycle arrest which occurs following exposure to radiation.